A nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, a dividing plate that cooperates with the tube sheet and channel head forming a primary fluid inlet header at one end of the tube bundle and a primary fluid outlet header at the other end of the tube bundle. A primary fluid inlet nozzle is in fluid communication with the primary fluid inlet header and a primary fluid outlet nozzle is in fluid communication with the primary fluid outlet header. The steam generator secondary side comprises a wrapper disposed between the tube bundle and the shell to form an annular chamber made up of the shell on the outside and a wrapper on the inside, and a feedwater ring disposed above the U-like curvature end of the tube bundle.
The primary fluid having been heated by circulation through the reactor core enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle the primary fluid is conducted through the primary fluid inlet header, through the U-tube bundle, out the primary fluid outlet header, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator secondary side through a feedwater nozzle which is connected to a feedwater ring inside the steam generator. Upon entering the steam generator, the feedwater mixes with the water returning from the separators, called the recirculation stream. This mixture, called the downcomer flow is conducted down an annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the water to reverse direction passing in heat transfer relationship with the outside of the U-tubes and up through the inside of the wrapper. While the water is circulating in heat transfer relationship with the tube bundle, heat is transferred from the primary fluid in the tubes to the water surrounding the tubes causing a portion of the water to be converted to steam. The steam then rises and is conducted through a number of moisture separators that separate entrained water from steam, and the steam vapor then exits the steam generator and is circulated through typical electrical generating equipment to generate electricity in a manner well known in the art.
Since the primary fluid contains radioactive materials and is isolated from the feedwater only by the U-tube walls, the U-tube walls form part of the primary boundary for isolating these radioactive materials. It is, therefore, important that the U-tubes be maintained defect-free so that no breaks will occur in the U-tubes that will cause radioactive materials from the primary fluid to enter the secondary side, an undesirable result.
Loose parts enter the steam generator through the feedwater stream and can cause damage to the heat transfer tubes. This damage can result in having to plug or repair the damaged tubes to avoid contamination of the secondary fluid. In extreme cases, the damage can lead to a tube leak and forced outage with significant expense to the plant. Therefore, it is important to prevent foreign objects from entering the steam generator and/or to remove the loose parts from the steam generator before tube damage occurs.
Prior attempts to prevent steam generator loose parts from reaching the tube bundle have focused on a sieving action. For example, spray nozzles with small holes have been attached to the feedwater distribution ring to trap loose parts. Although such spray nozzles have succeeded in trapping larger parts, small parts may pass through the holes in the nozzles due to their size. Such loose parts, e.g., pieces of metal rope or rods, have caused tube damage in operating steam generators.
Accordingly, it is an object of this invention to provide a loose parts collector that will remove from the feedwater introduced into the steam generator substantially all loose parts before the feedwater is placed in contact with the tube bundle.
Additionally, it is an object of this invention to provide such a loose parts collector that will not impede the efficiency of the steam generator.